1. Field of the Invention
The invention concerns a device made with thin layers of superconducting material and a method for its fabrication and, more particularly, a device with thin layers comprising at least one superconductive thin layer.
2. Description of the Prior Art
Josephson junctions are the basic constituent elements of numerous superconducting devices including SQUIDS (superconducting quantum interference devices), mixers, correlators, superconductive memories, etc).
The discovery of superconductivity at high superconducting transition temperatures (superconducting transition temperature hereinafter denoted Tc), in La.sub.2-x Sr.sub.x CuO.sub.4 type oxides (which have Tc close to the 30.degree. K.), and then in YBa.sub.2 Cu.sub.3 O.sub.7-y (which have Tc close to 90.degree. K.), has made it possible to envisage the making of Josephson junctions which have working temperatures in the region of the temperature of liquid nitrogen (77.degree. K.).
Various Josephson junctions technologies made with low Tc materials (materials such as Nb, Sn, Pb), have been previously studied (for example, point contact junctions, bridge junctions, micro-bridge junctions or junctions with variation in thickness). This invention concerns adaptation of the new "high Tc" materials, for junction technologies where there is a very thin layer of insulating, conductive or slightly superconductive material between the two electrodes. More precisely these junction technologies are superconductor-insulant-superconductor (SIS) tunnel effect junctions and sandwich junctions with proximity effect of the superconductor-normal metal-superconductor (SNS) type or the superconductor-lower Tc superconductor-superconductor (SS'S) type. FIGS. 1 and 2 give a schematic view of these different types of junctions.
FIG. 1 shows a tunnel effect junction with layers of superconductive materials S, S1 and S2 separated by an insulating layer I. The entire structure is supported on a substrate.
FIG. 2 shows a sandwich junction designed for proximity effect conduction, which has 2 layers of superconductive material, S1 and S2, separated by a normal metal layer N, or a lower Tc or a lower critical current (Jc) layer S'. A lower Tc and/or lower Jc makes the S' layers less superconductive than the materials of the layers S1 and S2. The structure is supported on a substrate.
The main constraint related to the use of these technologies with new materials lies in the fact that, to obtain the expected operation of the Josephson structure thus formed, it is absolutely necessary that the barrier between the two superconductive electrodes should have a thickness of the magnitude of the coherence length of the superconductive material used. This characteristic length was fairly high (280 angstroms for niobium, 2300 angstroms for tin, 830 angstroms for lead, etc.) in low Tc superconductors. In high to superconductors the coherence length is shorter. For instance, it is only 34 angstroms along the least favorable axis, namely c, in YBa.sub.2 Cu.sub.3 O.sub.7-y (and 7 angstroms along the axes a and b,), as determined recently by T. K. Worthington et al in "Physical Review Letters" 59 (10), 1160 (1987).
The superconductive character of the new oxide materials is intimately related to the structure, as has been demonstrated by several authors, especially R. J. Cava et al in Physical Review Letters 58 (16), 1676 (1987). It is therefore not only necessary to be able to make a layer with a thickness of about 20 angstroms in order to separate the two superconductive electrodes but also to have the ability, to obtain a material having the right crystal structure on either side of this barrier, to within a distance of about one elementary lattice constant.